The occurrence of fish hybrids in nature has been reported from time to time by several fishery workers. Slastenenko
(1957) has given a list of natural hybrids of the world which suggests that species belonging to the cyprinid group are
more prone to interbreed than other groups of fishes and Indian major carps belong to this group.

Natural occurrence of both interspecific and intergeneric hybrids of Indian major carps has been reported mostly
from reservoirs, formed due to the construction of dams across rivers. These hybrids were also reported from dry
“bundhs”. Occurrence of rohu-catla hybrid has been reported from the perennial irrigation tanks (reservoirs) in the
state of Madhya Pradesh in India. A detailed study of the taxonomic characters of this hybrid has been made by Desai
and Rao (1970). Occurrence of catla-rohu hybrids has been reported from Adhartal lake also in Madhya Pradesh. On
the basis of the morphological characters and position of the pectoral and anal fins, the parental species of these
putative hybrids have been identified as female catla and male rohu (Tripathi et al., 1974). Natural hybrids of similar
type have been also reported by Natarajan et al. (1976) from the Rehand reservoir in the state of Uttar Pradesh. The
impoundment with an area of 46,538 ha has a mean depth of 25m. The dominant ichthyofauna in the reservoir is
Catla catla which contributes as much as 98% of the fisheries (Anon, 1974).

The hybrids as reported by the authors possess similar traits but differing traits which reflect the broad spectrum of
character variations in hybrids. These natural hybrids as observed by Natarajan et al. (1976) have a general appearance
of C.catla with a conspicuously smaller head than catla but with a deeper body than rohu. The shape of the mouth was
more similar to rohu with infringed lips. The hybrid reported from Madhya Pradesh (Desai and Rao, 1970) on the
contrary had feebly fringed lower lip and the mouth showed one rudimentary maxillary pair of barbels.

The morphometric traits (ratios) of these natural hybrids more resembled catla in respect of length of head/diameter
of eye and length of fish/length of body, whereas, in respect of length of head/height of head ratio, the hybrid
resembled rohu, the intermediate traits being the ratio of length of fish/depth of body. The hybrid has also showed
intermediate traits in respect of meristic counts of dorsal and pectoral fin rays. Natarajan et al. (1976) have given the
full details of morphometric and meristic characters of these hybrids.

Occurrence of other carp hybrids like Labeo fimbriatus (Bl) x L. gonius (Ham.) and L.calbasu x C.catla in Rangwan
reservoir in Uttar Pradesh has been also reported by Prasad (1976). He suggested that these hybrids form good
substitutes for L. gonius and L. calbasu in reservoirs.

Similarly, the hybrids of catla-rohu and rohu-catla would also form a good substitute for rohu in reservoirs. Natarajan
et al. (1976) mentioned that the studies carried out in respect of the ecology and fisheries of Freshwater reservoirs
in Bhavanisagar (Tamil Nadu), Nagarjunasagar (Andhra Pradesh), Govindasagar (Himachal Pradesh), Rihand (Uttar
Pradesh) under the all India Co-ordinated Research Project and also D.V.C. reservoir (Jhingran and Natarajan, 1969)
and Tungabhadra (David et al., 1969) have shown that the performance of Labeo rohita in reservoir ecosystems was
not much satisfactory when compared to that of Catla catla, L. calbasu and Cirrhinus mrigala. The catla-rohu
hybrid, having wider spectrum than rohu was suggested to be a good substitute to rohu. Ceratium sp. constitutes a
very important fraction among phytoplankters in many reservoirs. This ecological niche is not utilized properly.
Introduction of Catla-rohu hybrid may help boost the fish production from reservoirs through the proper utilization
of the above mentioned niche (Natarajan et al., 1976).

Several causes have been suggested to account for the occurrence of hybrids in nature. Hubbs (1955) holds the view
that hybridization in nature is facilitated when there is scarcity of one species and dominance of an allied species in
close proximity.

In “bundh” breeding occurrence of natural hybrids may be due to congestion in the spawning ground. Because of the
limited space, there is very likelihood of ova of one species getting accidentally fertilized by the sperm of another
species. Particularly, in the case of Indian major carps with very compatible genomic structure (Zhang and Reddy,
1991), it is all the more easy and frequent to encounter natural hybrids when bred together in a relatively congregated
condition.

In any genetic improvement programme, it is necessary to be able to breed the candidate species artificially under
controlled conditions. Soon after the success of induced breeding of the Indian major carps, work on genetic
improvement of these carps was initiated in the late fifties starting with simple interspecific and intergeneric
hybridization to produce and evaluate their useful traits for aquaculture, since hybridization was regarded to be one
of the methods for combining desirable traits of selected species. Accordingly, hybridization work was carried out
between different species of a genus (interspecific) and species of different genera (intergeneric) among the Indian
major carps and Indian and Chinese major carps, including the common carp (Cyprinus carpio variety communis,
Linnaeus,) to study whether the hybrids acquired any useful traits from their respective parent species, such as
smaller head, wider body and more flesh content as in rohu-catla hybrid, pond breeding habit as in Cyprinus carpio,
or advantageous feeding habits. (Chaudhuri, 1959 and 1973; Alikunhi and Chaudhuri, 1959; Alikunhi et al., 1963a
and 1972; Naseem Humsa, 1971 and 1972. Naseem Hamsa and Alikunhi, 1971; Varghese and Sukumaran, 1971;
Ibrahim, 1977; Konda Reddy, 1977; Varghese and Shantharam, 1979; Basavaraju and Varghese, 1980a, 1980b; 1981,
1983; Basavaraju et al., 1989; 1990; 1994 and 1995; Ibrahim et al., 1980; Bhowmick et al., 1981; John and Reddy,
1986; Kesavanath et al., 1980; Konda Reddy and Varghese, 1980, 1980a, 1980b and 1983; Khan et al., 1989 and
1990; Jana, 1993). These experiments have demonstrated a high level of compatibility among Indian carps for
hybridization.

Altogether six interspecific and 13 intergeneric hybrids have been produced among the four species of Indian major
carps belonging to the three genera, viz. Catla, Labeo and Cirrhinus.

Some of the interspecific and intergeneric hybrids of Indian carps and also between Indian carps and common carp
(Cyprinus carpio var. Communis) which were observed to possess some useful traits from aquaculture point of
view are described here. Information regarding other hybrids involving Indian major carps is given in the Annex.

The percentage of fertilization was very high reaching about 94% and development of eggs was normal. The offspring
of both the hybrid crosses were highly viable and their growth rate was superior to their slow growing parent viz.
L. calbasu (Chaudhuri, 1971). Hybrids of both the crosses were found to be fertile and attained maturity within two
years. Plates 2 and 3.

Basavaraju et al. (1990) made a detailed investigation regarding the growth, food conversion ratio and protein
efficiency ratio of the hybrid produced between fimbriatus (female) and rohu male. The growth of the hybrid was
evaluated in comparison to the parent species. In an 182 day rearing experiment the hybrid was reported to attain
247.8g as compared to 324.5g in rohu and 161.7g in fimbriatus. The growth in the hybrid was 34.75% higher than
that of fimbriatus and 23.55 less than rohu. The hybrid also showed the highest survival, being 90% compared to
rohu with 86.67% and fimbriatus, 76.67%. In terms of percentage weight gain, specific growth rate and feed utilization,
the hybrid performed better than both the parental species.

The matured kalbasu-rohu hybrid has been bred through hypophyzation and the F2 generation was produced successfully
in 1960. As in the case of the F1 progeny, the percentage of fertilization in the F2 progeny was also recorded to be
high. The hybrids possessed varying characters which were intermediate between the original parents viz. Labeo
calbasu and Labeo rohita (Chaudhuri, 1971).

Of the 13 intergeneric hybrids produced (Table 5) only four hybrids, L. rohita-C. catla, Cirrhinus mrigala-C.catla,
L.rohita-Cirrhinus mrigala, L. fimbriatus-C. catla and the reciprocal hybrids have been found to possess useful
traits in terms of growth (Ibrahim, 1977; Basavaraju and Varghese, 1980 b). The following are the characteristics of
these hybrids.

Phenotypically the hybrid exhibits intermediate traits to that of the parents, but tending more towards the maternal
parent (L.rohita) with regard to body proportions and fin ray counts. Coloration of the body of the hybrid was near to
catla while the shape of the mouth was like rohu. However, its position with infringed lips was terminal and devoid of
barbels. The hybrid possessing a smaller head than that of catla and wider body than rohu shows improved traits over
the parents. The flesh content of the rohu-catla hybrid, 54–58%, has been also reported to be the highest when
compared to rohu with 48–52% and catla with 45% (Bhowmick et al., 1981 and Jana. 1993). Plate 4.

Growth

With regard to growth traits, the hybrid did not differ in growth from its parents in the initial stage. However, in the
grow-out stage the growth rate of the hybrid has been reported to vary. Chaudhuri (1973) observed the growth rate of
this hybrid was like that of catla, while Chaudhuri et al., 1975 reported, it to be 718–915g in one year duration in carp
polyculture experiments, whereas Bhowmick et al., 1981 observed it to be 900–1035g within the same duration.
Other workers, (Varghese and Shantharam 1979, Keshavanath et al. 1980, Konda Reddy and Varghese 1980,
Varghese et al. 1984, and Somalingam et al. 1990) have also reported that the rohu-catla hybrid grows better than rohu.

Konda Reddy and Varghese (1980b) conducted experiments on the growth performance of the rohu-catla and catla-rohu
hybrids along with the parent species. They observed that the rohu-catla hybrid (female rohu and male catla) showed
slightly faster growth than rohu but very much slower than catla.

Keshavanath et al. (1980), also evaluated the growth performance of the hybrids produced between L. rohita (female)
× C. catla (male) and the reciprocal hybrids along with their parental species under communal rearing and also
independently. These studies have shown that the growth of both the hybrids was inferior to catla and almost equal to
rohu. The growth difference was insignificant between the hybrids. However, when reared separately, the growth
performance of the hybrids was better than rohu but inferior to catla.

Varghese and Shantharam (1979) made a comparative study on the growth of three intergeneric hybrid of Indian
major carps viz., hybrids produced by crossing female rohu × male catla (rohu-catla), catla female × male mrigal
(catla-mrigal) and rohu female × mrigal male (rohu-mrigal). After 90 days of rearing, the rohu-catla hybrid showed
the fastest growth among the three hybrid types, followed by the catla-mrigal hybrid. Whereas, the rohu-mrigal
hybrid attained the lowest growth of all the three. In terms of percentage increase in the growth the rohu-catla and
the catla-mrigal have respectively shown 257.6% and 110.9% more over the rohu-mrigal hybrid.

According to the observations of Chaudhuri (1973) among the Indian carp hybrid rohu-catla was the best as it combined
the quick growth of catla and small head of rohu. He reported that the growth of this hybrid was faster than rohu and
nearly as fast as catla. However, the observations of Konda Reddy and Varghese (1980 b) have shown that this hybrid
grew slightly faster than rohu, but its growth when compared to catla was very poor. Somalingam et al. (1990)
studied the growth of rohu-catla hybrid stocked in three different water bodies, i.e., ponds and small and larger
reservoirs. The growth rate of the hybrid in ponds when compared to the parent species was 10–20% less than the
male parent catla and 30–125% more than the female parent rohu. However, in small reservoir the growth of the
hybrid during the second year of rearing equalled to catla though it was less by 20% and 32% during the first and third
years of rearing. At the same time, the hybrid has shown 53%, 128% and 32% higher growth over rohu during the
first, second and third year respectively. The authors suggested that the rohu-catla hybrid is a good replacement for
rohu to boost the production from the reservoirs.

A more detailed study, conducted at the fish farm of the Central Institute of Freshwater Aquaculture (CIFA),
Bhubaneswar in different combinations of the hybrid with the major carp species, i.e. catla, hybrid and mrigal; rohu,
catla and hybrid, and mrigal, rohu and hybrid have shown that the rohu-catla hybrid grew better than rohu and mrigal
(Jana, 1993).

The morphometric traits of rohu-catla and the reciprocal hybrids have been analyzed by different workers in comparison
to the parent species. According to Konda Reddy and Varghese (1980 a) the rohu-catla and catla-rohu hybrids exhibit
intermediate traits in respect of the morphometric ratios such as length of fish/length of head, length of head/snout
length, length of head/width of mouth and length of fish/length of dorsal fin. In other aspects like length of head/
interorbital space, the hybrids tend more towards catla. Rohu-catla hybrid resembled more of catla in the case of the
ratios of length of head/diameter of eye, while the reciprocal hybrid catla-rohu tends more towards rohu. The
taxonomic characters of these hybrids according to these authors with regard to meristic counts have been observed
to be similar, (data not shown).

Bhowmick et al. (1981) conducted significance test of rohu-catla hybrid for the fin ray counts and nine body
proportion ratios and compared them with the parental species rohu and catla. The values showed that the ratio of
body length/head width differ significantly from catla (P<0.01), whereas standard length/head length and standard
length/head width are significant at 5% level (P<0.05). Significant differences were also found for fin ray counts of
the pectoral fin (P<0.01). These authors have also stated that, although the hybrid appears to be intermediate in
general appearance to the parent species, the significance analysis clearly indicates that the hybrid has acquired more
of maternal (rohu) traits (data not shown).

Jana (1993), while working for his Ph.D. made a thorough investigation of the rohu-catla and catla-rohu hybrids with
regard to their biological and cytological aspects in comparison to their parental species. According to him the
morphometric characters of the hybrids and the parent species indicated that the mean values of trunk length and
body weight of the rohu-catla hybrid differed significantly (P<0.001). It was also observed that head height and body
height in trunk length and head width was significantly different (P<0.01) from catla.

Jana (1993) mentioned certain key characteristics of the rohu-catla and catla-rohu hybrids which may help to
identify these hybrids without much difficulty. In rohu-catla hybrid (rohu female and catla male) the pectoral fins
almost reaches the base of the pelvic fin which is a paternal parent trait in the case of catla, where the pectoral fin
almost touches the base of the pelvic fin unlike in rohu which leaves a larger gap between tip of pectoral fin when
extended towards the base of the pelvic fin. In the case of the reciprocal hybrid, catla female and rohu male (catla-rohu),
the pectoral fin when extended, does not touch the base of the pelvic fin and leaves some gap.

The compatibility in this reciprocal cross (catla-rohu) was moderately higher as indicated by the percentage of
fertilization and hatching, 65–80% and 60–80% respectively. The incubation period for the eggs to hatch out was
reported to be from 16.40 to 20.50 hrs after fertilization. The size of 72 hrs hatchling varied, from 6.8 to 7.2 mm.

Konda Reddy and Varghese (1983) produced this catla-rohu intergeneric hybrid and studied the embryonic and larval
developments and compared with the parental species. They reported that the developmental stages of the hybrid
were quite normal and no abnormalities were noticed and were similar to the parental species.

With regard to the external morphological features, both rohu-catla and catla-rohu hybrids were reported to show
similar intermediate traits. Catla has an upturned month with infringed lips while rohu has a ventrally pointed mouth
with fringed lips. However, both the hybrids possess a terminal mouth with slightly fringed lower lip. The hybrids
also possess a pair of maxillary barbels but shorter and not as prominent as in rohu. Konda Reddy and Varghese
(1983) and Jana (1993). Konda Reddy and Varghese (1980b) also reported that the mouth parts and gill rakers of
both the hybrids are very similar, indicating the same food habits. The observation of Alikunhi et al. (1971) have
shown the hybrid to grow better than even catla under monoculture as well as when grown with catla. Similarly,
Varghese and Sukumaran (1971) also observed the hybrid to grow better than even both the parents. This is in
contrary to what was reported by Konda Reddy and Varghese (1980 b). These workers reported that the catla-rohu
hybrid (catla female-rohu male) grew very much slower than catla and slightly slower than rohu. Rohu registered 25.2%
more than the hybrid while catla registered 108.9% more. These authors concluded that this hybrid is much inferior
to catla and slightly slower than rohu (Plate 5).

The reasons for this, according to Konda Reddy and Varghese (1980 b) seem to be that the food habits of these
hybrids as reported by Chaudhuri (1973) were basically detritophagic in pond environment and more like rohu. So
there appears to be food competition between the hybrids and rohu. Natarajan et al. (1976) also reported that in
reservoir environment the hybrid was detritophyto-planktophagic and has shown active inclination towards detritus
and phytoplankton in preference to zooplankton, which constitutes the chief food of catla.

This intergeneric hybrid cross exhibited relatively high compatibility as evidenced by the high rate of fertilization,
over 97%. The fully swollen and developing zygotes measured 5.8 mm while the hatchling measured over 4.0 mm,
which is much smaller than the catla-rohu hybrid.

Ibrahim (1977) made a thorough investigation of the hybrids produced between C. catla female and Cirrhinus
mrigala male and the reciprocal hybrids and also the hybrid between L. rohita female and C. mrigala male. According
to him the catla-mrigal hybrid had an elongated body with a terminal but slightly ventrally placed mouth. The upper lip
was slightly thick and the head more like catla. Pectoral, pelvic and anal fins were slightly pinkish but not as much as
in mrigal. The hybrid was devoid of barbels. The snout was slightly blunt tending towards the paternal parent, the
mrigal. Though not as pronounced as in catla, the hybrid had a slight hump. The body color was more like mrigal.
Diameter of the eye was larger in mrigal than in catla. But in the hybrid the eye was much larger than the parents.
Diameter of the eye in relation to head length was 6.89 mm in catla, 5.57 mm in mrigal and 4.129 mm in the hybrid.
From the view point of the morphometric characters, the hybrid differed from the parental forms with regard to fin
ray counts, dorsal, pectoral and pelvic fin rays, and tended more towards the maternal parent, the catla.

Food and feeding

Catla, a plankton and surface feeder has 245 gill rakers, which are slender, elongated and suitable for filtering,
whereas in mrigal, which is a bottom feeder as also mentioned earlier, has only over 60 short and slender gill rakers
which are placed widely. The food niches and food habits of the two parental forms are quite different. The hybrid
possesses 91 gill rakers, which is somewhat intermediate but nearer to the paternal parent, the mrigal. Thus the
hybrid though gained a little over mrigal in the filtering mechanism, did not gain over the mother.

Again, the hybrid with alimentary canal length of 6.15 times the total length, resembles the maternal parent, catla in
which it was 6.21 times. However, in mrigal it was much longer, being 18 times, the total length (Ibrahim 1977). Sex
ratio of the hybrids was 66.6% females and 33.4% males as reported by Ibrahim (1977).

Growth

Ibrahim (1997) made a detailed study on the growth of catla × mrigal hybrid and the reciprocal hybrids and compared
them with the parental species in communal ponds. According to him the catla × mrigal hybrid attained an average
growth of 107.7mm/10.3g in 11 months while the parental forms catla and mrigal attained 160.3mm/45.8g and
174.0mm/55.5g respectively. The growth of the hybrid, thus was not as good as the parental forms.

Unlike in the reciprocal hybrid, this hybrid possessed an up-turned mouth with the upper lip slightly over hanging.
Lower lip not fringed and without barbels. The hybrid was reported to resemble the catla (father) with regard to the
head and dorsal profile. The hump of catla was also clear in the hybrid. (Ibrahim, 1977).

The hybrid was found to be intermediate to the parents with regard to the anterior portion of the body in relation to its
total length. The depth of the body in the hybrid was greater than that of the mrigal as the former has inherited the
hump of catla. When compared to catla and rohu, mrigal has a minimum hump. The increase in the body depth due to
the hump can be regarded as a positive trait in the hybrid as in the case of the rohu-catla hybrid. The hybrid had an
increased size of head in relation to total length (4.58), compared to the mrigal (3.35) and smaller than in catla
(3.33). The eye diameter was relatively larger than in catla but not as large as in mrigal. The significant morphometric
characters are thus the depth of body and size of head of the hybrid which are positive gains (Ibrahim, 1977).

The fin ray counts in the hybrid with regard to the dorsal and pectoral fins, have shown differences from those of the
parents.

The pharyngeal complex in the hybrid with 144 gill rakers has shown significant deviation from both the parents
which, as mentioned earlier in mrigal was over 60 and 245 in catla. The increase in the number of gill rakers according
to Ibrahim, (1977) is an indication of the hybrid adoption towards plankton food habit as in catla.

Growth

The growth performance of this hybrid (mrigal × catla) according to Ibrahim (1977) was better than either parents.
The hybrid grew to an average size of 221.2mm/105g in 11 months compared to growth of catla and mrigal being
160mm/45.8g and 174mm/55.5g respectively.

The experiments of Ibrahim (1977) have shown that the hybrid cross had a very high percentage of fertilization
(97.9%) indicating the highly compatible nature between the genomes of these two intergeneric species. The
incubation period for the eggs to hatch was 15 hours and 30 minutes. The hatchlings, immediately after hatchling
measured 3.67mm. By about 92 hours the yolk sack in the hatchling (spawn) was completely absorbed. The hybrid
had a ventrally turned mouth with the upper lip over hanging. A maxillary pair of barbels was reported to be present.
The lower lip was not fringed as in the case of rohu (maternal parent). The hybrid possessed slightly greenish color
on the dorsal profile as in the case of rohu. However, the ventral side with silvery color resembled the paternal parent
(mrigal). Also, coloration of pectoral, pelvic and caudal fins resembled that of mrigal. The snout was slightly pointed,
resembling rohu.

The head length and body height of the hybrid were longer than the father. However, the hybrid showed lower gill-raker
length than both the parental. A positive trait in the hybrid was that it possessed a larger number of gill rakers
than both the parents. This according to Ibrahim (1977) is a probable indication of the hybrid tending towards
consumption of more planktonic food than either of the parents thus providing the hybrid a wider spectrum in
feeding. The sex ratio among the hybrids according to Ibrahim (1977) was 35% female, 65% male and some
maturing specimens of hybrids were noticed during the third year.

Growth

The growth of this rohu-mrigal hybrid was also good attaining an average size of 242.8mm/143g in 11 months.
Among these three intergeneric hybrids, catla-mrigal, mrigal-catla and rohu-mrigal, whose growth performance
was evaluated in communal pond along with the parental forms, catla, mrigal and rohu, the mrigal-catla and the
rohu-mrigal hybrids have shown a better overall growth performance than the parental forms (Ibrahim, 1977). However,
the author mentioned that the growth recorded in this evaluation should not be taken as a complete manifestation of
the growth potential as the fishes were confined in a small nursery, which is not considered ideal for maximum
growth assessment of fishes. However, the trend observed can be considered significant.

The overall analysis of morphometric and meristic characters of all these hybrids interestingly indicated that the
hybrids showed greater divergence from parental forms in characters like body height, head length, eye diameter, gill
rakers number and size, and also in the length of the alimentary canal (Ibrahim, 1977).

In the Cirrhinus mrigala (mrigal) × C. catla (catla), the body height had a gain of 15.8% against mrigal (mother)
while it had a loss of 16.5% over catla (father). In head size the hybrid gained 19.9% over mrigal and a loss of 27.6%
was observed over catla. In the length of gill rakers the hybrid gained 62.9% over mrigal and there was a loss of
63.3% over catla. Again in eye diameter the hybrid indicated a loss of 1.4% and a gain of 17.6% over the maternal and
paternal parental forms respectively.

Considering the other characters, this hybrid has a wider spectrum of feeding in converting more of column-planktonic
and bottom feeds. This, possibly could be the reason for a relatively better growth of the hybrid over the parental
forms (Ibrahim, 1977).

Basavaraju and Varghese (1980b) studied the growth rates of the L. rohita (female) × Cirrhinus mrigala (male) and
the reciprocal hybrids and compared them with that of the parental species which were also reared along with the
hybrids. This study has shown that both the parental species, L. rohita and Cirrhinus mrigala grew faster than both
the hybrids. Again, among the two hybrids the hybrid between Cirrhinus mrigala female × L. rohita male grew faster
than the reciprocal L. rohita female × C. mrigal male hybrid. Thus, the average weights attained by rohu, mrigal and
the reciprocal hybrids, mrigal-rohu and rohu-mrigal were 151.52, 92.37, 72.16 and 61.0g respectively. Whereas the
observations of Ibrahim (1977) have shown that growth rate of the rohu (female) × mrigal (male) was better than the
parent species.

Basavaraju et al. (1994) studied the morphometric and meristic characters of the hybrid between L. fimbriatus
female and C. catla male in comparison with the parental species. These authors stated that it was difficult to distinguish
these reciprocal hybrids.

The fimbriatus-catla hybrid also possessed intermediate characters to parental species in body profile, coloration of
fins and structure of lips. Body was deep with a smaller head than in the paternal parent catla. The Mouth was
subterminal with a moderately fringed lower lip.

With regard to the morphometric characters, the hybrid resembled catla (male parent) in total length/standard length,
length of head/eye diameter and length of head/height of head. Its resemblance with the female parent (fimbriatus)
was only in respect of head length/maximum head width. The intermediate characters were total length/fork length,
total length/pro-dorsal length and total length/least height of caudal peduncle (Basavaraju et al., 1994).

Growth

Evaluation of the hybrid between female C. catla and male L. fimbriatus (catla-fimbriatus) and the reciprocal hybrid
i.e. female L. fimbriatus and male C. catla (fimbriatus-catla) has been done by Basavaraju et al. (1995). These
workers reported that the growth of hybrids was much faster than the slow-growing parent, fimbriatus and less than
catla when assessed under similar conditions. On conclusion the growth rates recorded were, catla 64.2 fimbriatus
24g, hybrids fimbriatus catla 57.8g and catla-fimbriatus 39.8g.

The growth increment attained by fimbriatus-catla was 153.4 and 45.0% more than fimbriatus and the reciprocal
hybrid respectively and not much different from catla. As in the case of the rohu-catla hybrids, these hybrids also
exhibit intermediate traits with desirable qualities such as smaller head in the fimbriatus and deeper body as in catla.
The meat yield in the hybrids (62.1% in fimbriatus-catla and 63% in catla-fimbriatus) was higher than in the parents
(52.5% in catla and 61.9% in fimbriatus), as in the case of rohu-catla hybrid.

The hybrids between Indian major carps and common carp (Cyprinus carpio var. communis), involving the
crosses between common carp male × rohu female, common carp female × males of rohu, catla and mrigal in
each cross, respectively have on the whole exhibited normal fertilization and moderate viability, though in certain
cases, the crosses involving females of Indian carps with males of common carp, low viability has been
exhibited in the hybrids. Details of these intergeneric hybrid crosses are shown in Table 6.

Table 6. Intergeneric hybrids between Indian major carps and Common carp

FEMALE

MALE

HYBRID

COMMENTS

1. L.rohita × C. carpio

(rohu-common carp)

Hybrids exhibited poor survival and are sterile.

2. C. mrigala × C. carpio

(mrigal-common carp)

Poor survival. The hybrid tends to resemble C. carpio and are sterile.

3. C. carpio × L.rohita

(common carp-rohu)

Higher survival rate than the reciprocal-cross and dominated by mostly C. carpio traits. The hybrids are sterile (aneuploid), grow better under monoculture system.

4. C. carpio × C. catla

(common carp-catla)

Higher survival rate than the reciprocal-cross and dominated by mostly C. carpio traits. The hybrids are sterile (aneuploid), grow better under monoculture system.

Though the percentage of fertilization appeared to be good, the development in many of the eggs ceased during the
incubation period. The bulk of the embryos died during and after hatching and only a few hatchlings could grow up to
the adult stage which did not show any signs of maturity.

Morphologically, the hybrid appeared to be intermediate between the parents but statistical analysis of various
parameters showed it to be closer to the paternal parent (common carp).

Khan et al., 1990 made a detailed study of this hybrid cross between common carp female and L.rohita male. The
fertilization rate was as good as in pure common carp crosses. Hatching process was completed within 46.30 hrs at
water temperature 27–29.5° C.

The dorsally convex and elongated body of the hybrid was broader than that of rohu. In other morphological features
the hybrid had a terminal mouth which was also protractile as in common carp. The lips were not fringed but one pair
of maxillary and another pair of rostral barbels were present. Regarding the fins, the dorsal fin though long, but was
shorter than in common carp. Serrations of the spiny dorsal and anal fin rays which are very conspicuous in common
carp, were very feeble in the hybrid. The hybrid resembled the common carp in prepectoral distance, the lateral line
position, color of the body. However, the caudal fin in the hybrid was deeply forked with 24 rays. With regard
to size of scales, the hybrid was intermediate. The color of fins was also intermediate.

The characteristic big belly of common carp was absent in this hybrid, as in the case of mrigal-common carp
hybrid, and the shape of the abdomen resembles that of rohu, the paternal parent. The size of the head in the
hybrid was intermediate being smaller than that of common carp but bigger than in rohu. On the whole the
hybrid had more maternal traits. Plate 6.

The survival and growth of common carp-rohu hybrid and common carp under monoculture system were 91.45g and
73% and 76.5g and 71% respectively during a period of 180 days. However, when the hybrid was reared along with
the parents, common carp and rohu in equal ratios, the average growth performance and survival were 84.3g and
68.5%, 237g and 70% and 216.7g and 88% respectively. Thus the hybrid has shown slower growth and lower survival
than both the parents, when grown together.

Regarding food habits, these hybrids are bottom feeders as the maternal parent and their food items consisted of
semidigested crustacean appendages, bottom detritus and sand particles. Diatoms like Amphora, Navicula and
filamentus alga were also observed in the food. The dress value of the hybrid had a higher percentage of flesh
(58%) than that of common carp (47%), but less than in rohu (60.3%).

Regarding fertility, the hybrid did not exhibit any signs of maturity after two to four years of age. However, rudiments
of testes were observed in some individuals.

The cross of common carp female with male of C. catla showed a relatively high percentage of fertilization, ranging
from 65–80%, but in this case also about 35–45% mortality occurred during embryonic development, as in the case
of the mrigal-common carp hybrid. Most of the hatchlings were deformed with a curve in the caudal region and died
72 hrs after hatching.

The hybrid possessed a dorsally convex and more elongated body than common carp. A slight hump as in the case of
common carp was also present. As in the case of the common carp rohu hybrid, the characteristic belly of the
common carp was lacking in this hybrid too. The mouth of the hybrid was terminal and shorter than that of
common carp. Plate 7.

The hybrid attained an average weight of 21.0g in polyculture system and 71g under monoculture system (when
reared alone) within a period of two months (Khan et al., 1990).

Thus, over three decades of research on hybridization among different species of Indian major carps and their
evaluation has shown that these major carps though possessing distinct morphological characters, genera, they are
highly compatible and able to produce viable and fertile hybrid progenies. Matured interspecific and intergeneric
hybrids could be induced to produce F2 progeny or back-cross and triple cross hybrids.

The growth exhibited by most of these hybrids was intermediate i.e. better than the slow growing parent as in the case
of the interspecific hybrid between L. rohita and L. calbasu or the intergeneric hybrids between L.rohita and C.
catla or C. catla and Cirrhinus mrigala etc.

The gain in the growth in these hybrids over their slow growing parents appears to be due to the inheritance of this trait
from the fast growing parent, rohu in the case of L. rohita-L. calbasu hybrid and catla in the case of L.rohita-C. catla
hybrid and so on. Chevassus (1983) also expressed the opinion that the intermediate nature of the growth in the
hybrids, may be due to the partial transmission of this trait to the hybrid by the fast growing parent.

The growth of some hybrids like the rohu-catla or mrigal-catla or with their reciprocal cross appears to be due
to the available food in the medium and also could be due to the variations that probably existed in the rearing
medium under different agroclimatic zones.

With regard to the hybrid crosses between Indian major carps and Chinese grass carp, silver carp and big head carp,
no compatibility was observed in any of the hybrid progenies as almost all of them died either during the embryonic
development or soon after hatching, maximum within a week, except some stray hybrids produced between C. catla
and H. molitrix (silver carp).

Again, relatively better survival of the hybrid progeny was observed in the crosses between female common carp ×
males of Indian major carps, viz., C. catla, L. rohita and Cirrhinus mrigala compared to the reciprocal hybrid crosses
particularly between female L. rohita and male common carp. These reciprocal hybrids followed the maternal
parent in most of the embryonic developmental events particularly, duration of incubation, commencement of
cleavage etc. However, the reason the viability of hybrid progeny was poor in crosses where Indian major carps were
the maternal parents appears to be due to the haploid nature of most of those hybrid progenies as indicated by their
curved caudal region probably caused due to the relatively larger size of the head of the penetrating sperm of common
carp (with n=50–51 chromosomes) through the smaller micropyle of the major carp egg (with n=25 chromosomes).
Thus often preventing the genome input from the male and thereby proper fertilization.

However, further investigation may be necessary, especially on hybrids between Indian major carps and minor carps
and between Indian major carps and common carp.

It is essential to prevent indiscriminate hybridization by checking the mixed spawning of Indian carps. These carps, as
has been observed, are highly compatible to interbreed. Also, since almost all interspecific and intergeneric hybrids
were found fertile, we should be all the more cautious to check their cross-breeding which may cause serious
damage to the original gene pool through genetic introgression. (Padhi and Mandal, 1994, Mishra et al., 1998).

Some of these hybrids may be very useful for stocking reservoirs in India, where they are observed to thrive well and
contribute substantially to the production (Natarajan, et al., 1976).

Basavaraju and Varghese (1980) have made some study on organoleptic evaluation of the meat of rohu-mrigal and
mrigal-rohu and compared it to that of the parental species, rohu and mrigal. The test was done for both raw and
cooked meat. The attributes tested were color gloss of skin, odor of flesh, color of flesh, and texture for raw fish and
color, texture, taste and flavor for cooked meat. Analysis of variance (Krammer and Twigg, 1970, Basavaraju and
Varghese 1980) showed that the uncooked flesh of hybrids was found to be inferior in quality to that of the parental
species, while no significant difference was found with regard to the cooked flesh.

After the development of induced breeding technique for Indian major carps, production of pure seed of these carps
became possible. Dependence of the fish culturist for their seed from riverine sources gradually decreased for many
reasons. After the advent of the intensive carp culture system, more and more farmers started taking up fish culture.
Consequently there has been a growing demand for the seed of major carps. Based on the demand of fish farmers, the
seed producers that used Chinese-type hatcheries produced the seed of all three major carps, catla, rohu and mrigal,
by breeding them together in a common bedding pool through the administration of hormones. This practice is
mostly followed in the state of West Bengal where 70% of the carp seed requirement of the country is met (Padhi
and Mandal, 1994). This kind of seed production is generally resorted to by the seed producer, not only to supply
seed of all the three species simultaneously in the required quantities, but also to save time as the breeding span of
these carps lasts usually for only one and a half to two months. Thus the seed producer's aim is to supply the seed as
per the demand and earn as much money as possible, without realizing that there may be genetic consequences of the
practice.

Mishra. et al. (1998) have assessed the percentage of hybridization taking place in such mixed spawning practices.
According to their observations the rate of what they termed as spontaneous/inadvertent hybridization (intergeneric)
ranged from 7.24 to 9.24% in a fish seed hatchery, they studied in the state of Uttar Pradesh. The hybrids were
identified as catla-mrigal and, rohu-mrigal. The growth performance of these hybrids as observed by Mishra, et al.
(1998) was that the hybrids in general were found to possess significantly higher lengths and weights than their
parent species. The growth of catla-rohu was found to be highest among all. The rohu-mrigal hybrids performed
better than their parents. Performance of catla-mrigal hybrids was better than mrigal, but not catla.

Though all the three species bred together (mixed spawning), the incidence of hybridization (7.24 to 9.24%) is not
relatively very high in spite of the compatibility of these species to hybridize. The probable reasons for this could be
that Indian carps exhibit courting behaviour while spawning where the female is usually followed (chased) by several,
males very closely. Here, males might be following their like species during the chase, ultimately resulting in the
release of eggs by the female and simultaneously release of milt by the males.

It was also reported that the male coils round the abdomen of the female and releases milt when she releases the eggs
The spontaneous/inadvertent hybridization which is reported to take place during mixed spawn may be due to accidental
fertilization of the eggs of one species by the sperm of another species which might have been released in close
proximity.

Species specific courtship behaviour and generally species recognition keep the accidental level of hybridization
low. However, the presence of even a few number of hybrids among the lot of other pure species (Indian major
carps) may be detrimental to the genetic quality in the long run.